1. Field of the Invention
This invention relates to apparatus and an associated method for nondestructively determining the coating thickness on a metal surface, such as the interior of a metal can or a metal sheet.
2. Description of the Prior Art
It has been known to coat curved metal surfaces such as the interiors of cans in order to resist potentially damaging contact between the metal surface and materials which will be contained within or come into contact with the metal object. For example, in connection with aluminum cans used for food products, in order to resist undesired corrosive attack by the food products on the metal, or undesired chemical reactions which could alter the taste of the food products, the can interior is generally coated with an organic coating. It is important that the coating have the desired thickness in order to avoid providing an inadequate barrier due to an unduly thin coating, or interference with can forming techniques due to a coating which is of an excessive thickness. On metal sheet lubricating oil coatings may be present.
A number of means of coating such can interiors have been known, including various electrocoating processes. See generally U.S. Pat. Nos. 3,922,213, 4,094,760, 4,210,507, 4,400,251, 4,544,475 and 4,676,881.
It is has also been known to inspect coating thickness on can interiors by employing destructive testing techniques. These techniques involve periodically removing a sample can from the line and cutting it into pieces with coating thicknesses being measured on the sections. In connection with such destructive testing, various known means such as carbon analysis may be employed to ascertain coating thickness as a function of location within the can. Among the problems with destructive testing are the fact that destructive testing of this type is very time-consuming. By the time the results are available, a great number of containers may have been made with coated portions that are not within specifications. Also, even if the can passes the test, the destructive testing involved destruction of a good quality product.
U.S. Pat. No. 3,751,643 discloses the use of an spectrophotometer in measuring dielectric film thickness. This nondestructive-method involves monitoring variations in the interference effect caused by the difference in light reflected from the film and from the substrate are monitored. Computer means are employed to assist with computations.
It has been known to employ laser energy in measuring the thickness of a coating by monitoring radiant energy as compared with standard thickness specimens. See U.S. Pat. No. 4,634,291.
U.S. Pat. No. 4,656,358 discloses the use of ultraviolet laser beams in the nondestructive, noncontacting measurement of dimensions of sample wafers. It discloses the use of returning light solely from the spot on which the ultraviolet light impinges and the use of fluorescence created within the material being examined.
It has also been know to add materials to a coating in order to enhance fluorescence responsive to imposing a laser beam thereon. See U.S. Pat. Nos. 4,437,010, 4,460,274, 4,651,010 and German Patent 29 07 620.
U.S. Pat. No. 4,841,156 discloses the measurement of thickness of films employing the concept that the fluorescent light is proportional to a film thickness. The fluorescent light is filtered before being received by a photodetector which converts the light into an electrical signal. The system employees two signals and the use of a divider to compare the signals.
In spite of the foregoing known systems, there remains a very real and substantial need for a non-destructive, non-contacting system for accurately measuring the thickness of a relatively thin coating on a curved metal object, such as a metal can.